This invention relates to automatic implantable devices to detect and differentiate between tachycardias (rapid heart rhythms) in order to therapeutically stimulate the heart in response thereto, and more specifically, to distinguish hemodynamically stable and unstable ventricular tachycardias and provide appropriate treatments for the both.
Early automatic tachycardia detection systems for automatic implantable cardioverter defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intramyocardial pressure, blood pressure, impedance, stroke volume or heart movement) and or the rate of the electrocardiogram. For example, the 1961 pamphlet by Dr. Fred Zacouto, Paris, France, entitled, "Traitement D'Urgence des Differents Types de Syncopes Cardiaques du Syndrome de Morgangni-AdamsStokes" (National Library of Medicine) describes an automatic pacemaker and defibrillator responsive to the presence or absence of the patient's blood pressure in conjunction with the rate of the patient's electrocardiogram. Later detection algorithms proposed by Satinsky, "Heart Monitor Automatically Activates Defibrillator", Medical Tribune, 9, No. 91:3, Nov. 11, 1968, and Shuder et al "Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System", Transactions American Society for Artificial Internal Organs, 16:207, 1970, automatically detected and triggered defibrillation when the amplitude of the R-wave of the electrocardiogram fell below a predetermined threshold over a predetermined period of time. The initial system proposed by Mirowski et al in U.S. Pat. No. 27,757, similarly relied upon the decrease in the amplitude of a pulsatile right ventricular pressure signal below a threshold over a predetermined period of time, was abandoned by Mirowski et al in favor of the rate and/or probability density function morphology discrimination as described in Mower et al, "Automatic Implantable Cardioverter-Defibrillator Structural Characteristics", PACE, Vol. 7, November-December 1984, Part II, pp. 1331-1334. Others have suggested the use of high rate plus acceleration of rate "onset" (U.S. Pat. No. 4,384,585) with sustained high rate and rate stability (U.S. Pat. No. 4,523,595).
Very generally, the systems that depend upon the aforementioned criteria are capable of discriminating ventricular tachycardia in greater or lesser degree from normal heart rate but have difficulty discriminating sinus or other supraventricular tachycardias from malignant, pathologic ventricular tachycardias, resulting in inappropriate cardiac electrical stimulation.
As stated in the article "Automatic Tachycardia Recognition" by R. Arzbaecher et al, PACE, May-June 1984, pp. 541-547, anti-tachycardia pacemakers that were undergoing clinical studies prior to the publication of that article detected tachycardia by sensing a high rate in the chamber to be paced. The specific criteria to be met before pace termination was to be attempted involved a comparison of the detected rate to a preset threshold, such as 150 beats per minute (400 millisecond cycle length) for a preselected number of beats. As stated above, other researchers had suggested the rate of change of rate or suddenness of onset, rate stability and sustained high rate as additional criteria to distinguish sinus tachycardias from malignant ventricular tachycardias. Arzbaecher et al proposed in their article an algorithm implemented in a microprocessor based implantable device employing both atrial and ventricular rate detection via separate bipolar leads in order to detect the AA and VA, or VV and AV intervals (in "cycle lengths") against threshold intervals in order to distinguish among various types of tachycardias. The Arzbaecher et al article also discloses the concept of employing a single atrial extra stimulus to distinguish sinus tachycardia from 1:1 paroxysmal tachycardia. An atrial extra stimulus was delivered in late diastole (80 milliseconds premature), and if the ventricular response appeared early as well, sinus rhythm would be indicated. However, in tachycardias, such as AV reentrant and ventricular with VA conduction, the ventricular response would not occur early (indicating that the atrial and ventricular rhythms were disassociated) and the ventricular rhythm would be unperturbed.
Other proposals for employing atrial and ventricular detection and interval comparison are set forth in The Third Decade of Cardiac Pacing: Advances in Technology in Clinical Applications, Part III, Chapter 1, "Necessity of Signal Processing in Tachycardia Detection" by Furman et al (edited by S. Barold and J. Mugica, Futura Publications, 1982, pages 265-274) and in the Lehmann U.S. Pat. No. 4,860,749. In both cases, atrial and ventricular rates or intervals are compared to one another in order to distinguish sinus and pathological tachycardias.
The reliable distinction of sinus tachycardia from pathologic tachycardias remains a major limitation of even those systems and algorithms that have been proposed employing both atrial and ventricular detection and interval discrimination as described above. Many paroxysmal tachycardias are initiated by isolated premature beats. The tachycardias may reach a stabilized rate only after a number of beats. By contrast, some sinus tachycardias may mimic paroxysmal tachycardias in their abrupt initiation. Moreover, ventricular tachycardias can be either hemodynamically stable or unstable. Hemodynamically stable ventricular tachycardias exhibit monomorphic constant cycle lengths without loss of consciousness. Hemodynamically unstable ventricular tachycardias may also exhibit constant cycle lengths but result in loss of consciousness and therefore require a more rapid termination, for example by means of a cardioversion pulse. Stable tachycardias do not require termination as quickly, allowing for use of less aggressive treatments such as antitachycardia pacing to attempt termination before employing cardioversion shocks.
Accordingly, it is an objective of the present invention to provide an improved medical device for treating ventricular tachyarrhythmias, including ventricular tachycardia, flutter, and fibrillation with improved techniques for distinguishing hemodynamically stable ventricular tachycardias from hemodynamically unstable ventricular tachycardias and applying appropriate therapies.